This invention relates to automatic gain control circuits for single sideband receivers and, in particular, to automatic gain control circuits for single sideband receivers that have included therein a noise floor automatic gain control circuit which maintains the output background noise level at a substantial percentage below the output level of a strong signal.
There is no unique feature associated with single sideband voice communication signals to provide reliable squelch operation. The skilled operators of High Frequency (HF) radios do not normally use the prior art type squelch because of the danger of missing usable communications. If the Automatic Gain Control (AGC) is allowed to govern the audio output signal level, the noise level will be brought up to the normal speech level when the channel is idle. This high idle channel noise level is not only fatiguing to the operator, but the high acoustic noise level limits the number of receivers a single operator can monitor to two or three. The skilled HF radio operator therefore adjusts the radio frequency (RF) gain control of the radio receiver so that the AGC can no longer bring the background noise level up to the normal speech level. The output of the receiver now increases with signal level for some 20-30 dB above the noise level until the input signal level is at a high enough level to activate the AGC circuit. This procedure reduces operator fatigue and allows the operator to monitor three or four receivers simultaneously. Manual RF gain control adjustments are generally made hourly in order to maintain the background noise level at an acceptable level. However, there are no reliable, automatic, comparable AGC techniques known in the prior art that provide the volume control needed by unskilled HF operators, especially where the users have other functions to perform in addition to HF communications.